Vacuum decarburization in rh and dh type degassing systems

ABSTRACT

A method for cyclically decarburizing a bath of molten metal such as steel by transferring the molten metal from a first vessel to a second vessel, evacuating the space above the molten metal in the second vessel while subjecting the metal bath in the second vessel to oxygen to decarburize the same, and returning the decarburized metal from the second to the first vessel. The foregoing cycle is repeated while the pressure within the second vessel is progressively reduced and the oxygen introduced into the second vessel varied to decarburize the molten metal bath in steps without substantial oxidation of metal constituents therein.

United States Patent [191 Ramachandran et a].

[451 Mar. 19, 1974 VACUUM DECARBURIZATION IN RH AND DH TYPE DEGASSING SYSTEMS [73] Assignee: Allegheny Ludlum Industries, Inc.,

Pittsburgh, Pa.

22 Filed: Dec. 29, 1971 21 Appl. No.: 213,528

[52] U.S. Cl 75/49, 75/46, 75/60 [51] Int. Cl. C2lc 7/10, C2lc 5/28 [58] Field of Search 75/49, 60, 46

[5 6] References Cited UNITED STATES PATENTS 3,380,509 4/1968 Hentrich 75/49 X 2,893,860 7/1959 Lorenz 75/49 3,042,510 7/1962 Armbruster et a1. 75/49 3.320.053 5/1967 Lehman 75/49 X 3,606,293 9/1971 Erdely 75/49 X 2,093,666 9/1937 Vogt 75/49 X 3.594.155 7/1971 Ramachandran 75/49 X FOREIGN PATENTS OR APPLICATIONS 1,238,058 6/1960 France 75/49 1,014,827 12/1965 Great Britain..... 75/49 1,912,907 9/1970 Germany 75/49 Primary Examiner-Charles N. Lovell Assistant ExaminerPeter D. Rosenberg Attorney, Agent, or FirmVincent G. Gioia [5 7] ABSTRACT A method for cyclically decarburizing a bath of molten metal such as steel by transferring the molten metal from a first vessel to a second vessel, evacuating the space above the molten metal in the second vessel while subjecting the metal bath in the second vessel to oxygen to decarburize the same, and returning the decarburized metal from the second to the first vessel. The foregoing cycle is repeated while the pressure within the second vessel is progressively reduced and the oxygen introduced into the second vessel varied to decarburize the molten metal bath in steps without substantial oxidation of metal constituents therein.

6 Claims, 3 Drawing Figures Pmimmumsmn 3.798025 SHEEF 1 0F 2 FIG. /.7

VACUUM DECARBUlRIZATION IN RH AND DH TYPE DEGASSING SYSTEMS BACKGROUND OF THE INVENTION While not limited thereto, the present invention is particularly adapted for use in decarburizing steels, especially stainless steels. This is achieved with the use of so-called DH and RH type vessels which are normally used for vacuum degassing. In the DH process for degassing, an upper vacuum vessel is lowered until a nozzle or snorkel extending downwardly from its bottom is introduced into a molten metal bath within a ladle. Vacuum is applied to the upper vessel such that metal is sucked into this vessel from the metal bath below, the amount of metal sucked into the vessel being controlled by the depth of immersion of the snorkel in the molten metal bath and the pressure (below atmospheric pressure) within the vacuum vessel. After the metal in the vacuum vessel is degassed, it is emptied by raising the vessel and the snorkel, permitting the degassed metal to flow back into the lower ladle. A new batch of molten metal is then brought into the vacuum chamber by lowering the vessel. The geometry of the vessel and ladle systems is such that satisfactory operation of the system is only possible with the chamber evacuated to a medium to low pressure.

The RH process uses a vacuum chamber which is fitted with two legs or snorkels which are immersed in a melt carried within a lower ladle. The bottom of the chamber is sloped with the inlet leg located at a higher elevation than the outlet. Part way in the inlet leg, provision is made for the insertion of inert gas. This gas operates as a gas lift device which pumps molten metal from the lower ladle into the vacuum vessel. As in the DH system, a low pressure is required within the vacuum vessel in order to elevate the metal to the level where the gas lift device is located.

It will be readily appreciated that the amount of metal drawn into the vacuum chamber in both the DH and RH processes is controlled by the vacuum level in the vessel. That is, more metal is sucked into the treating vessel as the vacuum level drops. This is desirable during degassing since normal degassing is more efficiently accomplished at low pressures. However, when decarburizing under vacuum conditions, it is desirable to conduct the process at variable pressures is explained, for example, in Ramachandran US. Pat. No. 3,594,155. This is particularly true in the case of stainless steels where the process must be controlled precisely to avoid oxidation of chromium values. Hence, normal DH and RH vessels, as presently constituted, are not practical for use in decarburizing stainless steel because they are dependent upon the vacuum level within the treatment chamber in order to suck metal into that treatment chamber.

SUMMARY OF THE INVENTION In accordance with the present invention, a method is provided, using a modified form of the DH and RH systems, which enables the decarburization of steels at any pressure from atmospheric pressure down to low vacuums. Specifically, there is provided means, other than the vacuum level within the treating chamber itself, for causing molten metal to flow from a lower ladle up into the treatment chamber and thereafter be returned to the treatment chamber. In one embodiment of the invention shown herein, the basic Dl-I configuration is used; however instead of sucking molten metal up into the treatment chamber, the lower ladle containing the molten bath to be treated is surrounded by an air-tight chamber and air pressure used to force the molten metal upwardly through the treatment chamber snorkel. 1

In another embodiment of the invention using the basic RH configuration, air pressure can be used to force molten metal upwardly through both the entry and discharge legs of the vacuum vessel or by means of an electromagnetic pump coil surrounding the entry leg.

In both systems, whether the basic DH or RH system is employed, provision is made for the injection of oxygen into the vacuum chamber. Preferably, this is accomplished with the use of tuyeres projecting through the wall of the vacuum chamber and arranged such that they will be beneath the level of molten metal in the vacuum chamber during treatment.

Further, in accordance with the invention, the vacuum level within the treatment chamber is progressively increased (i.e., the pressure is decreased) as successive portions of the molten metal bath within the ladle are decarburized such as to achieve the minimum carbon level in the decarburizing vessel without oxidation of the metallic elements. In the DH system, the pressure is varied in steps for each batch of motlen metal brought upwardly into the vacuum vessel; whereas in the RH process, the pressure can be varied in steps or continually, depending upon requirements.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. 1 is a schematic illustration of one embodiment of the invention utilizing the basic DH system;

FIG. 2 is a schematic illustration of another embodiment of the invention utilizing the basic RH system; and

FIG. 3 is a plot showing the manner in which carbon content is reduced using the DH system.

With reference now to the drawings, and particularly to FIG. I, there is shown a reaction apparatus in accordance with the invention based upon the DH process. The system includes a ladle 10 containing a bath 12 of molten metal to be decarburized, such as stainless steel. Above the ladle I0 is a vacuum reaction vessel 14 having a downwardly-extending nozzle or snorkel 16 extending into the molten metal bath 12. The reaction vessel 14 can be provided with an upper addition hopper 18 from which additives are added to the melt within the reaction vessel 14. The interior of the reaction vessel 14 is connected through conduit 20 to apparatus for producing controlled vacuum conditions, such as a vacuum pump or steam ejectors. Projecting through the wall of the vessel 14, near the bottom thereof, is a tuyere 22 through which oxygen can be injected into the molten metal within the reaction vessel 14 during decarburization. Surrounding the ladle 10 is a closed chamber 24 connected through conduit 26 to a source of pressure such that the pressure above the surface of the molten metal bath 12 can be varied. As will be understood, the chamber 24 can be opened by suitable means to permit the ladle to be removed and a new ladle inserted.

In operation, the interior of the chamber 24 will be pressurized via conduit 26, thereby forcing the liquid metal bath 12 upwardly through the snorkel 16 into the reaction vessel 14. At the same time, the area above the molten metal in the reaction chamber 14 is evacuated through conduit 20, and oxygen is injected into the bath in the reaction chamber 14 to decarburize the steel contained therein. Following this procedure, the pressure within chamber 24 is reduced, whereupon the portion of the bath in the vessel 14 drains down into the ladle 10. Then a new batch is forced upwardly into the reaction chamber 14 by again pressurizing the chamber 24; and this procedure is repeated, with portions of the molten metal bath being decarburized in succession, until a desired carbon content is reached in the bath 12. As the pressure in vessel 14 is reduced after each cycle, so also is the pressure in chamber 24 to maintain a constant level in vessel 14.

If desired, an induction heating coil may be provided around the reaction vessel 14 in order to add heat to the melt therein. However, in most cases, this will not be required since the introduction of oxygen into the molten metal within the vessel 14 will generate heat. Additionally, an induction coil can be used around the vessel 14 in order to effect stirring of the melt during the decarburization cycle. Again, however, by proper placement of the oxygen injection tuyere 22, sufficient mixing will result without the necessity for inductive stirring.

While a tuyere 22 has been shown in the embodiment of the invention of FIG. 1, it will be appreciated that oxygen can also be introduced using a lance which blows oxygen onto the surface of the metal bath within the vessel 14. Preferably, the tuyere or tuyeres 22 are placed at a level such that during the normal decarburization cycle, the metal level in the reaction vessel 14 is increased to the point where the tuyeres become submerged. However, when the metal within the reaction vessel 14 is returned to the ladle 10, the tuyere or tuyeres will be exposed. In this way, there is no need to keep the tuyeres open, if the apparatus is used for degassing, by flushing with an inert gas.

As is explained in Ramachandran U.S. Pat. No. 3,594,155, it is necessary to lower the pressure in the vacuum system as the carbon concentration in the metal bath decreases. This can be done continuously or in steps; however there are some important differences, which are best illustrated by outlining the procedures on a step-by-step basis.

if it is assumed that a batch of molten metal from the bath 12 has been forced upwardly into the vessel 14 by pressurizing chamber 24, the oxygen is forced through the tuyeres 22, thereby causing oxidation of carbon and gas stirring. It will be assumed that the carbon concentration of the metal at the start of the decarburization cycle is about 0.4% carbon. it will also be assumed that the tuyeres are immersed to a depth of about 24 inches, that the melt temperature is 2,900 F and that the metal composition is 18.75% chromium, 8.5% nickel, and 0.1% silicon. It will also be assumed for illustration that a four-stage steam ejector is used to evacuate the vessel For the particular analysis and temperature given above, the minimum carbon levels that can be reached without chromium losses can be calculated from the following equations:

where C weight %C at end of nth cycle, in the ladle; FT fraction treated; and C weight %C reached in chamber at end of treatment cycle. Note that these are rigorous formulae, noting that l0O/100-FT (C C E 1.0

Note

1[1 FTP FT C,,=(l -FT)" C [l (1 FT)" FT/FTJ FT -C =(1 FT)" C,,+[1+ FT- (1 FT)] C The resulting data for computed values of C are shown in the following Table 1:

TABLE 1 Minimum Carbon Concentration That can be Reached Without Metallic Oxidation of an 18.75% Cr, 8.50% Ni, 0.1% Si Melt at 2900F as Predicted by Decarburization Model Assumptions: Buhblc diameter 0.5 cm., depth of injection 24 inches, oxygen.

Assuming that the fourth-stage ejector alone can operate efficiently down to millimeters of mercury, it can be seen that the carbon level can be brought down to about 0.16% carbon and the carbon level can be reduced to about 0.078% using the fourth and third stage ejectors which operate efficiently from 150 to millimeters of mercury. If it is assumed that the end point carbon desired is around 0.08%, it can be seen that only the fourth and third-stage ejectors need be used.

The process is schematically illustrated in FIG. 3. Between times t, and a first batch of metal is decarburized in the vessel 14. During this time, the carbon concentration within the vessel 14 decreases along the broken line 30. When this decarburized batch of metal is then returned to the ladle 10, the carbon concentration in the ladle 10 decreases from C, to C Between times I2 and a second batch of metal is forced upwardly into the reaction vessel 14 and subjected to oxygen. Again, the carbon concentration in the batch being treated decreases along broken line 32 from carbon level C to some lower level; whereupon the treated batch is returned to the ladle 10 and the carbon concentration of the total mass of metal then falls to level C The cycle is repeated a number of times, in this case six times, until the total carbon concentration in the bath is reduced to C,,. Assuming a four-stage steam ejector is used to create the vacuum, only the fourth stage must be used during the first three cycles; however in the fourth, fifth and sixth cycles, when the pressure is reduced further, both the third and fourth stages must be used.

One advantage of the DH system is that the carbon removal process can be accomplished rapidly. That is, if the rate of carbon removal is plotted against carbon content, it will be seen that the removal rate is relatively constant at a high value until the carbon content reaches a predetermined level, below which the removal rate falls off rapidly. In decarburizing the melt in batches, each of which has a relatively high carbon content, the decarburization of each batch can be carried out in the range of high carbon removal rate. These individual batches, when returned to the ladle 10, then serve to lower the carbon content of the total bath to that level where the removal rate is low.

With reference now to FIG. 2, an embodiment of the invention is shown wherein an RH type system is employed. In this case, an upper reaction chamber 36 is provided with two downwardly-extending snorkels 38 and 40 which extend beneath the surface of a molten metal bath 42 contained within a ladle 44. In the usual case, the snorkel 40 is provided with an inlet 43 for the injection of an inert gas. The upper reaction vessel 36 is lowered until the ends of the two snorkels 38 and 40 are submerged beneath the molten metal bath in the ladle 44, after which the reaction vessel 36 is evacuated via conduit 45 which leads to a pump or steam ejectors. As in the embodiment of FIG. 1, a chamber 46 surrounds the ladle 44 and is connected through conduit 48 to a pump, not shown, which pressurizes the area within the chamber 46. In this manner. the combined effect of the pressure within chamber 46 and the vacuum created above the bath in the vessel 36 causes the liquid metal in bath 42 to rise through the snorkels 38 and 40 into the reaction vessel 36. Argon or another inert gas can be injected via inlet 43 to decrease the density of the column of liquid metal in snorkel 40 and cause a pumping action resulting from imbalance between that column and the denser column of liquid metal in the other extension. Alternatively, an inductive coil 50 can be placed around the snorkel 40 to cause the liquid metal to rise therein.

In the embodiment of FIG. 2, the molten metal more or less continuously circulates through the reaction vessel 36; and as it passes through the reaction vessel 36, oxygen is injected via tuyere 52. As the decarburizing process continues, the pressure within the reaction vessel 36 is gradually reduced below atmospheric pressure. At the same time, the pressure within chamber 46 is reduced to maintain a more or less constant level in the vessel 36. The minimum carbon level which can be reached without chromium oxidation at each pressure level should be increased by a value equal to that carbon loss expected in the metal stream, assuming that all of the oxygen in the input gas is consumed as carbon monoxide. Again, the process is controlled in accordance with the teachings of US. Pat. No. 3,594,155.

Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes in process steps may be made to suit requirements without departing from the spirit and scope of the invention.

We claim as our invention:

1. In the method of decarburizing a bath of molten metal, the steps of l transferring a portion of the molten metal in a first vessel to a second vessel, (2) evacuating the space above said portion of the molten metal in said second vessel to achieve a first predetermined pressure below atmospheric pressure, (3) subjecting the metal while in said second vessel to oxygen to oxidize the carbon content thereof while preventing substantial oxidation of metallic elements in the molten metal, (4) returning said portion of the molten metal from the second vessel to the first vessel, repeating steps (I) to (4) above until the carbon content of said bath of molten metal has been reduced to a desired level and progressively reducing the pressure in said second vessel below atmospheric pressure in successive repetitions of step (2) of the method.

2. The method of claim 1 wherein a batch of molten metal is transferred to the second vessel, subjected to oxygen, and then returned to the first vessel to complete a cycle, and wherein the pressure is reduced in said second vessel in steps and at the completion of each cycle.

3. The method of claim 2 wherein said second vessel is above the first vessel and connected thereto through a single snorkel which extends into a molten metal bath contained in said first vessel.

4. The method of claim 3 including the step of pressurizing the surface of the metal bath in said first vessel to force metal from the first vessel upwardly through said snorkel into the second vessel.

5. The method of claim 4 wherein the pressure on the surface of the metal bath in the first vessel is reduced as the pressure in the second vessel is reduced.

6. The method of claim 1 wherein the molten metal continuously flows from said second vessel to said first vessel and is continually returned to said first vessel, and wherein said pressure is continually reduced in said second vessel as the metal moves from the first vessel to the second vessel and then back to the first vessel, until a desired carbon level is reached in said molten metal bath. 

2. The method of claim 1 wherein a batch of molten metal is transferred to the second vessel, subjected to oxygen, and then returned to the first vessel to complete a cycle, and wherein the pressure is reduced in said second vessel in steps and at the completion of each cycle.
 3. The method of claim 2 wherein said second vessel is above the first vessel and connected thereto through a single snorkel which extends into a molten metal bath contained in said first vessel.
 4. The method of claim 3 including the step of pressurizing the surface of the metal bath in said first vessel to force metal from the first vessel upwardly through said snorkel into the second vessel.
 5. The method of claim 4 wherein the pressure on the surface of the metal bath in the first vessel is reduced as the pressure in the second vessel is reduced.
 6. The method of claim 1 wherein the molten metal continuously flows from said second vessel to said first vessel and is continually returned to said first vessel, and wherein said pressure is continually reduced in said second vessel as the metal moves from the first vessel to the second vessel and then back to the first vessel, until a desired carbon level is reached in said molten metal bath. 